1. Field of the Invention
This invention relates to a method for the supervision of the quality of molded articles produced by injection molding and particularly to a method for the supervision of injection molding from the outset of the filling of molten resin into a mold to the completion of the molding on the basis of waveform of the inner die pressure in a resin passageway in the mold.
2. Description of the Prior Art
As one conventional supervising method of this type, there has been proposed an apparatus for the correction of molding data obtained during injection molding disclosed in Japanese Patent Application Public Disclosure Sho. 57(1982)-212041. Although the prior art apparatus can observe an occurrence of abnormality every shot of injection molding and a variation in the condition of the respective shots of injection molding, it nevertheless cannot find out what process is abnormal at one shot of injection molding and what may be the cause. With the prior art apparatus, the cause of abnormality in molding can therefore be investigated as a result of compiling statistics of the entire molding condition of several molding shots carried out for fixed times.
Further, the prior art apparatus, as claimed in the disclosure, utilizes the resin pressure in a mold cavity as one of variables in the molding process. However, the idea of determining reference, allowable and critical values of the resin pressure at one molding shot is not disclosed at all. Besides, the term "resin pressure" used therein is generally termed as "peak pressure", but does not signify internal die pressure described later.
Furthermore, there has been proposed in the disclosure a method of displaying on a cathode-ray tube device the deviation value of variables obtained over the entire molding lots. However, the injection molding as a continuing process from the outset of the filling of molten resin to the completion of the molding cannot be indicated for every shot of injection molding.
Still more, the aforementioned apparatus does not serve a function of comparing each molding shot with the optimum reference shot which is experimentally determined in advance.
Now, in keeping with the development of engineering plastics of today, injection molding is naturally required to be accomplished with reproduction on the order of microns and to promptly resume the optimum molding condition thereof when an accident occurs.
In order to meet the former requirement, it is necessary to judge abnormality at each shot of injection molding and further find out what process is abnormal in the injection molding. This is because the quality of molded articles depends practically on various factors over several molding processes from the outset of the filling process for injecting molten resin into the mold to the termination of the dwelling process, and therefore, supervision of examining a molded article to see whether it is good or not cannot promptly be accomplished with accuracy unless the process in which the abnormality is caused is found out. Accordingly, the standpoint of supervision is the primary requisite to examine the quality of the molded articles.
One of the present inventors of this invention previously accomplished a method of enabling resin pressure in a mold to be sized as a continuous waveform of internal die pressure of molten resin filled in the mold, which is obtained on the basis of functions of properties of plastics as disclosed in Japanese Patent Application Public Disclosure Sho. No. 52(1977)-14658. The inner die pressure represents the conditions of the molten resin from the outset of the filling process to the termination of the dwelling process and is detected by means of a sensor disposed in a resin passageway for introducing molten into the mold. In this sense, the internal die pressure in the resin passageway should be distinguished from the resin pressure considered as a peak pressure which is detected by means of a sensor disposed in a cavity of the mold as disclosed in Japanese Utility Mode Publication Sho. No. 39(1964)-18976 and it is expressed as pressure waveform in the form of an analogue. That is to say, the waveform of the internal die pressure shows the variation in condition of one molding shot, under which a molded article is produced and is referred to as the result of each molding condition. Accordingly, the supervision of the quality of molded articles can be carried out by use of the aforementioned waveform of the internal die pressure.
In order to comply with the latter of the requirements so far described, it is desired to compare each shot of injection molding with the reference shot under observation, thereby promptly detecting what process differs from that under the optimum condition. Consequently, the cause of the deviation in the molding process can be estimated from the abnormal portion thus recognized.